The intervertebral disc functions to support and transfer large multi-directional loads and permit motion of the spine. With degeneration, the mechanical function of the disc is compromised through a cascade of structural and compositional degradation. These changes in internal load distribution may have dramatic effect on the internal disc stresses and strains. Unfortunately, because the disc is an enclosed system, very little is known about internal disc deformations. In Aim 1 we propose to noninvasively quantify internal strains of human intervertebral discs using MRI while under physiological loads. Discectomy, the removal of NP tissue, is a widely applied treatment for herniation and low back pain. However, discectomy decreases the nuclear pressure, and as in natural degeneration, may lead to higher AF strains, create new tears throughout the disc, and accelerate the degenerative process. In Aim 2, we propose to noninvasively quantify internal disc strains after discectomy. While the structural mechanical studies in Aims 1 and 2 will provide critical new information about internal disc strains, in order to understand, predict, prevent, and treat disc degeneration it is critical to develop a comprehensive knowledge of the mechanical capability of the tissue-level substructures. The AF withstands a complex multi-axial loading environment, however, despite this recognized complexity, most of the current knowledge is based on uniaxial loading. At the tissue level, biaxial testing more appropriately mimics AF in vivo loading conditions. We have developed a fiber-reinforced constitutive model and demonstrated that fiber-matrix interactions contribute to AF nonlinear mechanical behavior. In Aim 3 we propose to measure the two-dimensional planar biaxial mechanics of the human AF, and apply a nonlinear, anisotropic model to quantify the role of the fibers, matrix, and fiber-matrix interactions in nondegenerate and degenerate AF. Successful completion of these aims will increase the understanding of the internal function of the disc. The MRI strain analysis techniques can be used in future studies of disc treatments. Furthermore, these results will provide critical structural-level internal strains and tissue-level constitutive formulations, which can be applied to determine the functional effects of degeneration and treatments on spine mechanics.